It is thus a recognised fact, or general law of nature, that nothing can act or move without undergoing some change, however trifling in amount. Not even a breath of wind can pass along the surface of the earth without altering in some degree the proportions of the bodies with which it comes into contact; and not a drop of rain can fall upon a stone without carrying away some portion of its substance. The smoothest and most accurately formed wheel, running along the most level and polished railroad, parts with some portion of its substance at every revolution, and in process of time is worn out and requires to be replaced. The same effect is forcibly, though rather ludicrously, exemplified in the great toe of the bronze statue of St Peter at Rome, which in the course of centuries has been worn down to less than half its original size by the successive kisses of the faithful; and I venture to mention it, because it affords one of the best specimens of the operation of a principle, the existence of which, from the imperceptibly small effect of any single act, might otherwise be plausibly denied. As regards _dead_ or _inanimate_ matter, the destructive influence of action is constantly forced upon our attention by every thing passing around us; and so much human ingenuity is exercised to counteract its effects, that no reflecting person will dispute the universality of its operation. But when we observe shrubs and trees waving in the wind, and animals undergoing violent exertion, for year after year, and yet both continuing to increase in size, we may be inclined, on a superficial view, to regard _living_ bodies as constituting exceptions to the rule. On more careful examination, however, it will appear that waste goes on in living bodies not only without any intermission, but with a rapidity immeasurably beyond that which occurs in inanimate objects. In the vegetable world, for instance, every leaf of a tree is incessantly pouring out some portion of its fluids, and every flower forming its own fruit and seed, speedily to be separated from and lost to its parent stem; thus causing in a few months an extent of waste many hundred times greater than what occurs in the same lapse of time after the tree is cut down, and all its living operations are at a close. The same thing holds true in the animal kingdom. So long as life continues, a copious exhalation from the skin, the lungs, the bowels, and the kidneys, goes on without a moment’s intermission; and not a movement can be performed which does not at least partially increase the velocity of the circulation, and add something to the general waste. In this way, during violent exertion several ounces of the fluids of the body are sometimes thrown out by perspiration in a very few minutes; whereas, after life is extinguished, all the excretions cease, and waste is limited to that which results from ordinary chemical decomposition.